Field of the Invention
The invention relates to a heat exchanger and to a motor vehicle HVAC system.
Description of the Background Art
Vehicle HVAC systems are used to heat and/or cool air supplied to the interior of a motor vehicle. In vehicle HVAC systems, heat exchangers are used as electrical heating devices to heat the air supplied to the interior. The electrical heating device comprises PTC elements. PTC elements (PTC: Positive Temperature Coefficient) are current-conducting materials that have an electrical resistance and can conduct current better at lower temperatures than at higher temperatures. Their electrical resistance therefore increases with increasing temperature. The PTC element generally is formed of ceramic and is a thermistor. Independent of the boundary conditions, such as, e.g., applied voltage, nominal resistance, or volume of air at the PTC element, a very uniform surface temperature arises at the PTC element. Overheating can be prevented as could occur, e.g., with a heating wire emitting normal heat, because here independent of the boundary conditions approximately the same resistance and thereby a substantially identical electrical heat output are always applied.
The heat exchanger comprises PTC elements, at least two electrical conductors by means of which electric current is conducted through the PTC element, and heat-conducting elements, particularly lamellae or corrugated fins, by means of which the surface for heating air is increased. Motor vehicles are produced increasingly, which have an exclusively electric drive or a hybrid drive. Vehicle HVAC systems for these motor vehicles generally no longer have a heat exchanger for heating air through which cooling fluid flows. The total heat output of the vehicle HVAC system for this reason must be delivered by the electrical heating device or the PTC elements. For this reason, it is also necessary to operate the PTC elements with high voltage, e.g., in the range of 50 to 600 V, instead of low voltage with 12 V. High voltage in a vehicle HVAC system is a safety problem, however, because, for example, if a person comes into contact with parts under high voltage, injuries may be inflicted on his health due to the high voltage.
U.S. Pat. No. 4,327,282 shows a heat exchanger with a PTC heating element. Current is conducted through the PTC heating element by means of contact plates and an insulating layer is arranged at the contact plates. The components are held together by a U-shaped clip.
EP 1 768 458 A1, which corresponds to U.S. Pat. No. 7,676,144, discloses a heat-producing element of a heating device for heating air, comprising at least one PTC element and electrical strip conductors that rest against opposite side surfaces of the PTC element, whereby the two electrical strip conductors are surrounded on the outside by an electrically non-conductive insulating layer.